Reduced-tantalum superalloy composition of matter and article made therefrom, and method for selecting a reduced-tantalum superalloy

ABSTRACT

A superalloy article has a composition consisting essentially of, in weight percent, from about 4 to about 12 percent cobalt, from about 3.5 to about 7 percent tungsten, from about 2 to about 9 percent chromium, from about 0.5 to about 4.5 percent tantalum, from about 5.5 to about 7.5 percent aluminum, from 0 to about 5.5 percent rhenium, from about 0.1 to about 1.2 percent titanium, from 0 to about 3 percent molybdenum, from 0 to about 3 percent ruthenium, from about 0.5 to about 2 percent columbium, about 0.01 percent maximum boron, about 0.07 percent maximum carbon, from about 0.3 to about 1 percent hafnium, about 0.01 percent maximum zirconium, about 0.03 percent maximum yttrium, from 0 to about 0.5 percent vanadium, about 0.01 percent maximum cerium, and about 0.01 percent maximum lanthanum, balance nickel and impurity elements. The article is preferably substantially a single crystal or oriented polycrystal in a shape such as a gas turbine blade.

[0001] This invention relates to a composition of matter suitable foruse in aggressive, high-temperature gas turbine environments, andarticles made therefrom.

BACKGROUND OF THE INVENTION

[0002] Nickel-base superalloys are alloys having more nickel than anyother element, and containing a group of elements that producegamma-prime and related precipitates during an appropriate heattreatment. Nickel-base superalloys are the currently preferred alloychoice for making the components of aircraft-gas turbine engines thatare exposed to the highest temperatures. Examples include turbineblades, turbine vanes, some shafts, some rotors, interstage seals, andmany high-temperature stationary gas-path components.

[0003] The nickel-base superalloys must exhibit acceptable mechanicalproperties at both low and high temperatures, such as good strength,good fatigue resistance, low creep rates, sufficient ductility, andacceptable density. They must also have good corrosion and oxidationresistance in the harsh combustion-gas environment. Further, thesuperalloys must have good stability in both extended exposure atelevated temperature and cyclic heating and cooling patterns. Theseproperties are achieved through the careful selection of the alloyingelements and the processing of the material. A number of superalloycompositions have been developed to supply the appropriate combinationsof these properties for various applications in the gas turbineenvironment.

[0004] Additionally, the cost of the superalloy material is aconsideration. While achieving the required properties is of paramountconcern, the manufacture and sales of gas turbine engines is acompetitive business. Some of the elements used in the nickel-basesuperalloys are rather exotic in nature and costly, and therefore theirpresence and amount is an important factor in the cost of the gasturbine engine. Further, some elements are subject to periodic shortageswherein the price becomes almost prohibitively high.

[0005] In the work leading to the present invention, the inventorsrecognized that one such element that is used in advanced nickel-basesuperalloys is tantalum. An important nickel-base superalloy used in gasturbine blades and other applications, Rene™ N5, contains a nominal 6.2weight percent tantalum, and other nickel-base superalloys contain 5percent or more of tantalum. In the last several years, the price oftantalum of the quality required for use in nickelbase superalloysincreased from about $100 per pound to $475 per pound, with someprojections of even higher price based on worldwide shortages. Othereconomic forces have temporarily lowered the price, but there is afuture potential for comparable price increases and shortages. The highprices and potential shortages thus threaten the continued economicviability and availability of articles made of such materials.

[0006] There is accordingly a need for improved nickel-base superalloyswith properties comparable with existing high-tantalum nickel-basesuperalloys such as Rene™ N5, but which are not as dependent upon theuse of high percentages of tantalum. The present invention fulfills thisneed, and further provides related advantages.

SUMMARY OF THE INVENTION

[0007] The present invention provides a nickel-base superalloy andarticles made from the nickel-base superalloy, and an approach forselecting and designing nickel-base superalloys. The nickel-basesuperalloy contains a reduced nominal tantalum content as compared withhigher-tantalum alloys, and with corresponding modifications of otheralloying elements to provide the comparable performance of thehigher-tantalum alloys.

[0008] An article comprises a composition consists essentially of, inweight percent, from about 4 to about 12 percent cobalt, from about 3.5to about 7 percent tungsten, from about 2 to about 9 percent chromium,from about 0.5 to about 4.5 percent tantalum, from about 5.5 to about7.5 percent aluminum, from 0 to about 5.5 percent rhenium, from about0.1 to about 1.2 percent titanium, from 0 to about 3 percent molybdenum,from 0 to about 3 percent ruthenium, from about 0.5 to about 2 percentcolumbium, about 0.01 percent maximum boron, about 0.07 percent maximumcarbon, from about 0.3 to about 1 percent hafnium, about 0.01 percentmaximum zirconium, about 0.03 percent maximum yttrium, from 0 to about0.5 percent vanadium, about 0.01 percent maximum cerium, and about 0.01percent maximum lanthanum, balance nickel and impurity elements. Mostpreferably, the article includes from about 3.0 to about 4.0 percenttantalum.

[0009] In one preferred form, the article includes from about 3.0 toabout 4.0 percent tantalum, from about 0.2 to about 0.4 percenttitanium, from about 0.5 to about 0.7 percent hafnium, and from about 1to about 2 percent columbium. In another preferred form, the articleincludes from about 6 to about 12 percent cobalt, from about 4.5 toabout 6.5 percent tungsten, from about 5.5 to about 6.5 percentchromium, from about 3.0 to about 4 percent tantalum, from about 5.8 toabout 6.3 percent aluminum, from about 2.8 to about 3.5 percent rhenium,from about 0.2 to about 0.4 percent titanium, from about 1.3 to about1.7 percent molybdenum, from about 0.5 to about 0.7 percent hafnium, andfrom about 1 to about 2 percent columbium. In a most-preferred form, thearticle includes from about 7 to about 10 percent cobalt, from about 6to about 6.3 percent tungsten, about 6 percent chromium, from about 3.1.to about 3.5 percent tantalum, from about 5.9 to about 6.3 percentaluminum, about 0.3 percent titanium, about 0.6 percent hafnium, about 3percent rhenium, about 1.5 percent molybdenum, and about 1.5 percentcolumbium.

[0010] Desirably, the article is substantially a single crystal or adirectionally oriented polycrystal produced by directionalsolidification. It is preferably shaped as a component of a gas turbineengine, such as a gas turbine blade.

[0011] A related composition of matter consists essentially of, inweight percent, from about 4 to about 12 percent cobalt, from about 3.5to about 7 percent tungsten, from about 2 to about 9 percent chromium,from about 0.5 to about 4.5 percent tantalum, from about 5.5 to about7.5 percent aluminum, from 0 to about 5.5 percent rhenium, from about0.1 to about 1.2 percent titanium, from 0 to about 3 percent molybdenum,from 0 to about 3 percent ruthenium, from about 0.5 to about 2 percentcolumbium, about 0.01 percent maximum boron, about 0.07 percent maximumcarbon, from about 0.3 to about 1 percent hafnium, about 0.01 percentmaximum zirconium, about 0.03 percent maximum yttrium, from 0 to about0.5 percent vanadium, about 0.01 percent maximum cerium, and about 0.01percent maximum lanthanum, balance nickel and impurity elements.Preferred and most-preferred compositions as discussed elsewhere hereinare applicable to the composition of matter.

[0012] The present invention also provides an approach for extending theprinciples used to develop the above-described composition to themodification of other nickel-superalloys to reduce their tantalumcontents. A method for selecting a reduced-cost nickel-base superalloycomprises the steps of identifying a baseline nickel-base superalloyhaving a nominal composition, in weight percent, comprising a baselinetantalum content of more than about 5 weight percent tantalum, and abaseline sum (baseline hafnium content plus baseline columbium contentplus baseline titanium content plus baseline tungsten content), inweight percent. (Calculated quantities are enclosed in parentheses forclarity herein.) The method further includes selecting a modifiednickel-base superalloy having a nominal composition, in weight percent,comprising a modified tantalum content at least 1.5 weight percent lessthan the baseline tantalum content, and a modified baseline sum of(modified hafnium content plus modified columbium content plus modifiedtitanium content plus modified tungsten content) at least 1.5 weightpercent greater than the baseline sum.

[0013] That is, the reduction in tantalum content for cost reasons mustbe compensated for by increasing the sum of hafnium, columbium,titanium, and tungsten. Preferably, the increase in the sum of hafnium,columbium, titanium, and tungsten is at least as great as the decreasein the tantalum content. That is, the absolute value of (the modifiedbaseline sum minus the baseline sum) is preferably at least as great asthe absolute value of (the modified tantalum content minus the baselinetantalum content). It is also preferred that the modified nickel-basesuperalloy have a nonzero modified hafnium content, a nonzero modifiedcolumbium content, a nonzero modified titanium content, and a nonzeromodified tungsten content. Desirably, the sum of the modified tungstencontent plus a modified molybdenum content is at least about 6.5 weightpercent, for most modified nickel-base superalloys.

[0014] Commercial baseline nickel-base superalloys such as PWA 1484,Rene™ 142, and the CMSX alloys such as CMSX-4 and CMSX-10 may bemodified according to these principles to reduce their tantalum contentswhile maintaining acceptable properties.

[0015] The present article, especially in its preferred andmost-preferred forms, exhibits performance comparable with that ofhigher-tantalum alloys, but with a reduced tantalum content, andconsequently a reduced cost. The cost savings becomes highly significantwhen tantalum prices exceed several hundred dollars per pound, as hasbeen the case recently and which may occur again in the future. Otherfeatures and advantages of the present invention will be apparent fromthe following more detailed description of the preferred embodiment,taken in conjunction with the accompanying drawings, which illustrate,by way of example, the principles of the invention. The scope of theinvention is not, however, limited to this preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a perspective view of a gas turbine blade;

[0017]FIG. 2 is a block flow diagram of a method for fabricating thearticle of FIG. 1; and

[0018]FIG. 3 is a bar chart of oxidation weight loss in cyclic oxidationtests, for the tested alloys.

DETAILED DESCRIPTION OF THE INVENTION

[0019]FIG. 1 depicts a component article 20 of a gas turbine engine,illustrated as a gas turbine blade 22. The gas turbine blade 22 includesan airfoil 24, an attachment 26 in the form of a dovetail to attach thegas turbine blade 22 to a turbine disk (not shown), and a laterallyextending platform 28 intermediate the airfoil 24 and the attachment 26.In one preferred embodiment, the component article 20 is substantially asingle crystal. That is, the component article 20 is at least about 80percent by volume, and more preferably at least about 95 percent byvolume, a single grain with a single crystallographic orientation. Theremay be minor volume fractions of other crystallographic orientations andalso regions separated by low-angle boundaries. The single-crystalstructure is prepared by the directional solidification of an alloycomposition as discussed herein, usually from a seed or other structurewhich induces the growth of the single crystal and single grainorientation. In another preferred embodiment, the component article 20is a directionally oriented polycrystal, in which there are at leastseveral grains all with a commonly oriented preferred growth direction.The directionally oriented polycrystal is produced by directionalsolidification, typically without a seed.

[0020] The use of the alloy composition discussed herein is not limitedto the gas turbine blade 22, and it may be employed in other articlessuch as gas turbine vanes, or articles that are not to be used in gasturbine engines.

[0021]FIG. 2 is a block-flow diagram of a preferred approach forpracticing the invention. The alloying elements that form thenickel-base alloy are provided in the proper proportions and meltedtogether to form the molten alloy, step 40. In a specific form, thealloy has a composition consisting essentially of, in weight percent,from about 4 to about 12 percent cobalt, from about 3.5 to about 7percent tungsten, from about 2 to about 9 percent chromium, from about0.5 to about 4.5 percent tantalum, from about 5.5 to about 7.5 percentaluminum, from 0 to about 3.5 percent rhenium, from about 0.1 to about1.2 percent titanium, from 0 to about 3 percent molybdenum, from 0 toabout 3 percent ruthenium, from about 0.5 to about 2 percent columbium,about 0.01 percent maximum boron, about 0.07 percent maximum carbon,from about 0.3 to about 1 percent hafnium, about 0.01 percent maximumzirconium, about 0.03 percent maximum yttrium, from 0 to about 0.5percent vanadium, about 0.01 percent maximum cerium, and about 0.01percent maximum lanthanum, balance nickel and impurity elements. (Allcompositional percentages herein are stated in weight percent, unlessindicated to the contrary.)

[0022] Preferably, the alloy includes from about 3.0 to about 4.0percent tantalum. In one preferred form, the article includes from about3.0 to about 4.0 percent tantalum, from about 0.2 to about 0.4 percenttitanium, from about 0.5 to about 0.7 percent hafnium, and from about 1to about 2 percent columbium. In another preferred form, the articleincludes from about 6 to about 12 percent cobalt, from about 4.5 toabout 6.5 percent tungsten, from about 5.5 to about 6.5 percentchromium, from about 3.0 to about 4 percent tantalum, from about 5.8 toabout 6.3 percent aluminum, from about 2.8 to about 3.5 percent rhenium,from about 0.2 to about 0.4 percent titanium, from about 1.3 to about1.7 percent molybdenum, from about 0.5 to about 0.7 percent hafnium, andfrom about 1 to about 2 percent columbium. In a most-preferred form, thearticle includes from about 7 to about 10 percent cobalt, from about 6to about 6.3 percent tungsten, about 6 percent chromium, from about 3.1.to about 3.5 percent tantalum, from about 5.9 to about 6.3 percentaluminum, about 0.3 percent titanium, about 0.6 percent hafnium, about 3percent rhenium, about 1.5 percent molybdenum, and about 1.5 percentcolumbium.

[0023] Most high performance superalloys for the most-demandingapplications contain at least about 5-6 weight percent tantalum, and insome cases considerably more tantalum. The present invention desirablyreduces the tantalum content of the alloy of the invention to no morethan about half the initial amount in the baseline nickel-basesuperalloy, and typically less than about 4 weight percent. Theresulting superalloy, with adjustments of other alloying proportions,has acceptable performance and also a cost which is significantly lessthan that of comparable superalloys, an important consideration at timeswhen the cost of tantalum is high.

[0024] The tantalum in the gamma-prime hardened superalloy is animportant ingredient because tantalum is a heavy refractory which canreplace aluminum in the Ni₃Al-based gamma-prime strengthening phase. Thetantalum has a secondary effect of improved castability with respect tograin defects by balancing out the density differences between the firstand last liquid to solidify (i.e., between the dendrite core and theinterdendritic regions). The presence of tantalum also does not have anegative effect on environmental resistance in respect to oxidation andhot corrosion, unlike other refractory metals such as molybdenum andtungsten. Thus, reducing the tantalum content below about 5 weightpercent, while retaining strength and environmental-resistanceproperties is challenging.

[0025] In the present approach, tantalum may be replaced by columbiumand/or hafnium and/or titanium and/or tungsten on the gamma-primealuminum sites. The tungsten partitions to both the gamma phase and tothe gamma prime phase, so that it aids in increasing the strength of thegamma prime phase as well as the gamma matrix. However, an excessivelylarge increase in the tungsten content tends to lead to phaseinstability in the alloy over long exposure to elevated temperature, andtherefore the tungsten content is limited.

[0026] The present alloy contains from about 0.5 to about 4.5 percenttantalum, more preferably from about 3 to about 4 percent, and mostpreferably from about 3.1 to about 3.5 percent. If the tantalum contentis less than about 0.5 percent, the alloy has insufficient strength. Ifthe tantalum content is less than about 2.5 percent, the strength isunsatisfactory for many applications, and the article is prone tocasting defects. If the tantalum content is more than about 4.5 percent,the alloy cost becomes prohibitive as the cost of tantalum increases.Also, a tantalum content of greater than about 4 percent, with thepresent levels of columbium, titanium, and hafnium, produces anickel-base superalloy with too much gamma prime phase and resultinginstability.

[0027] The alloy contains from about 0.1 to about 1.2 percent titanium.Titanium is a potent gamma prime hardener, and at least about 0.1percent must be present in order to compensate for the reduced tantalumcontent. The optional titanium addition substitutes for aluminum andtantalum in the gamma prime phase, improving the strength. However,higher levels of titanium adversely affect oxidation resistance.

[0028] The alloy contains from about 0.3 to about 1 percent hafnium.Hafnium improves the oxidation and hot corrosion resistance of coatedalloys, but can degrade the corrosion resistance of uncoated alloys.Hafnium also improves the life of thermal barrier coatings, where used.Experience with other alloys has shown that hafnium contents on theorder of 0.75 percent are satisfactory. However, when the hafniumcontent exceeds about 1 percent, the stress rupture properties arereduced and the incipient melting temperature is reduced.

[0029] The alloy contains from about 0.5 to about 2 percent columbium(also sometimes termed “niobium”), which substitutes for tantalum in thegamma prime phase. Lesser amounts result in insufficient amounts andstrength of the gamma prime phase. Greater amounts excessively reducethe gamma-prime solvus temperature and reduce oxidation resistance.

[0030] The alloy contains from about 4 to about 12 percent cobalt.Lesser amounts result in reduced alloy stability. Greater amounts reducethe gamma prime solvus temperature and thus the high-temperaturestrength, and impair the oxidation resistance.

[0031] The alloy contains from about from about 3.5 to about 7 percenttungsten. Lesser amounts unacceptably decrease the strength of thesuperalloy, and greater amounts produce instability with respect to TCP(topologically close packed) phase formation.

[0032] The alloy contains from about 2 to about 9 percent chromium.Lesser amounts reduce hot corrosion resistance while greater amountslead to phase instability and poor cyclic oxidation resistance.

[0033] The alloy contains from about 5.5 to about 7.5 percent aluminum.Lesser amounts reduce strength due to a reduction in the gamma primephase. Greater amounts produce instability with respect to TCP phaseformation and incipient melting problems during alloy heat treatment.

[0034] The alloy contains from 0 to about 5.5 percent rhenium, morepreferably from 0 to about 3.5 percent rhenium, even more preferablyfrom about 2.8 to about 3.5 percent rhenium, and most preferably about 3percent rhenium. Greater amounts produce alloy instability with respectto TCP phase formation.

[0035] The alloy contains from 0 to about 3 percent ruthenium. Greateramounts reduce oxidation resistance and do not improve alloy stability.

[0036] The alloy contains about 0.01 percent maximum boron, preferablyabout 0.006 percent maximum boron. Greater amounts cause incipientmelting problems during alloy heat treatment.

[0037] The alloy contains about 0.07 percent maximum carbon. The carbonis a deoxidizer to reduce inclusions. Greater amounts sap the strengthof the superalloy by chemically combining to form carbides of hardeningelements. The carbides also serve as the sites for fatigue failureinitiation.

[0038] The alloy contains about 0.01 percent maximum zirconium. Greateramounts cause incipient melting problems during alloy heat treatment.

[0039] The alloy contains about 0.03 percent maximum yttrium. Greateramounts promote undesirable mold-metal reaction at the casting surfaceand increase the inclusion content of the cast article.

[0040] The alloy contains from 0 to about 0.5 percent vanadium. Greateramounts reduce the hot corrosion resistance of the alloy.

[0041] The alloy contains about 0.01 percent maximum cerium and about0.01 percent maximum lanthanum. Greater amounts of either of theseelements promote an undesirable mold-metal reaction at the castingsurface and increase the inclusion content of the component.

[0042] The alloy preferably contains about 0.1 percent maximum silicon.Silicon in such minor amounts may aid oxidation resistance.

[0043] The alloy preferably contains about 0.04 percent maximummagnesium and about 0.01 percent maximum calcium as de-oxidizers. Theseelements in small quantities may also improve the oxidation resistance.

[0044] The balance of the alloy is nickel and impurity elements. Thenickel content is preferably in the range of from about 61 to about 64weight percent.

[0045] Studies and calculations were performed to establish limits forthe various elements. The following Table I sets for the compositions ofalloys actually melted. Alloys E1-E18 are alloys within the scope of thepresent invention, and alloy RN5 is commercial Rene™ N5 alloy, which isnot within the scope of the invention. TABLE 1 No. Al Ta Cr W Cb Co TiHf Y Ni E1 6.25 3.5 6 5 1 10 0 0.15 0.015 63.5 E2 6.25 3.5 6 6 1 10 00.15 0.015 62.5 E3 6.25 3.5 6 5 1.5 10 0 0.15 0.015 63.0 E4 6.25 3.5 6 61.5 10 0 0.15 0.015 62.0 E5 6.25 3.5 6 5 1 10 0 0.6 0.015 63.1 E6 6.253.5 6 6 1 10 0 0.6 0.015 62.1 E7 6.25 3.5 6 5 1.5 10 0 0.6 0.015 62.6 E86.25 3.5 6 6 1.5 10 0 0.6 0.015 61.6 E9 6.25 3.5 6 6 1 10 0.3 0.15 0.01562.2 E10 6.25 3.5 6 5 1.5 10 0.3 0.15 0.015 62.7 E11 6.25 3.5 6 5 1 100.3 0.6 0.015 62.8 E12 6.25 3.5 6 6 1.5 10 0.3 0.6 0.015 61.3 E13 6.223.5 6 6.5 1.5 10 0 0.15 0.015 61.5 E14 6.22 3.5 6 6.5 1.0 10 0 0.6 0.01561.5 E15 6.25 4.0 6 5.5 1.3 10 0 0.15 0.015 62.1 E16 6.60 3.5 6 5.5 1.010 0.3 0.15 0.015 62.4 E17 6.20 3.5 7 5 1.5 10 0.3 0.15 0.015 61.8 E186.20 3.5 7 5 2.0 10 0.3 0.15 0.015 61.3 RN5 6.2 6.5 7 5 0 7.5 0 0.15 063.1

[0046] For these alloys, in all cases the Mo content was 1.5 weightpercent, the Re content was 3 weight percent, the Ru content was 0, andthe carbon content was 0.05 weight percent.

[0047] Compositional and property computed values for the alloys are setforth in Table II. The value of ΔTa is the change in tantalum contentfor the indicated alloy as compared with RN5. The value of Δ(Ti+Hf+Cb+W)is the change in the computed sum for the indicated alloy as comparedwith RN5. The value of (W+Mo) is the numerical sum of these twoelements. TABLE II No. ΔTa Δ(Ti + Hf + Cb + W) (W + Mo) Density E1 −3.01.0 6.5 0.309 E2 −3.0 2.0 7.5 0.311 E3 −3.0 1.5 6.5 0.310 E4 −3.0 2.57.5 0.311 E5 −3.0 1.5 6.5 0.309 E6 −3.0 2.5 7.5 0.311 E7 −3.0 2.0 6.50.310 E8 −3.0 3.0 7.5 0.311 E9 −3.0 2.3 7.5 0.310 E10 −3.0 1.8 6.5 0.309E11 −3.0 1.8 6.5 0.309 E12 −3.0 3.3 7.5 0.311 E13 −3.0 3.0 8.0 0.313 E14−3.0 2.9 8.0 0.312 E15 −2.5 1.8 7.0 0.311 E16 −3.0 1.8 7.0 0.308 E17−3.0 1.8 6.5 0.309 E18 −3.0 2.3 6.5 0.309 RN5 0 0 6.5 0.312

[0048] Creep Rupture tests were performed for these alloys. Thetemperatures, times, and number of hours to failure are shown in TableIII: TABLE III 2100° F., 2000° F., No. 11 ksi 18 ksi 1800° F., 35 ksi1600° F., 75 ksi E1 20.0 32.7 69.8 40.8 E2 47.3 114.4 118.1 123.9 E340.9 61.1 114.6 95.9 E4 67.6 100.9 147.1 162.3 E5 23.7 42.1 76.9 48.4 E634.1 68.1 99.7 79.1 E7 52.5 68.6 120.6 95.3 E8 75.1 97.3 120.4 139.5 E942.7 80.4 105.2 102.1 E10 34.3 53.3 88.6 40.0 E11 61.6 58.2 106.0 81.8E12 217.3 165.9 132.9 205.9 E13 42.7 83.7 123.5 100.2 E14 46.7 87.9102.9 75.0 E15 61.9 82.7 130.5 211.8 E16 180.9 98/76.5 127.5 174.4 E1733.0 62.5 96.5 93.5 E18 86.3 85.6 118.5 174.7 RN5 104.3 147 150.5 182.2

[0049] The approach taken in the alloy development was to replacetantalum with columbium and/or hafnium and/or titanium and/or tungstenon gamma prime aluminum sites, and to provide tungsten for additionalgamma solid solution strengthening. A slight chromium reduction was madeto offset the tungsten increase to maintain alloy stability. The alloycompositions set forth in Table I were evaluated. The compositionsE1-E12 represent two designed experiments, to establish the effects oftungsten, columbium, hafnium, and titanium modifications. Alloys E1-E8are a full factorial in tungsten, columbium, and hafnium, while alloysE1, E4, E6-7, and E9-12 are a 2⁴⁺¹ _(IV) experiment to economicallyunderstand the effects of titanium modifications. Alloys E8 and E12 havethe same tungsten, columbium, and hafnium contents, but alloy E12 has0.3 percent titanium, with the result that the mechanical performance ofalloy E12 is substantially improved over that of alloy E8.

[0050] Small lab scale heats were vacuum melted for each composition.The melts were subsequently directionally solidified into columnargrained specimens to form directionally oriented polycrystals and testedin the longitudinal direction. Because the grain boundaries are parallelto the stress direction in the testing, the effect of the grainboundaries is minor. Alloy RN5 has the nominal composition of Rene™ N5alloy.

[0051] Based upon the testing, composition E12 was selected as thepreferred alloy composition, and acceptable variations were defined forspecific applications as set forth above. Some specific alloys ofinterest include: TABLE IV Alloy No. Al Ta W Co Hf Y-1716 6.25 3.5 6.010 0.60 Y-1717 6.25 3.5 6.0 7.5 0.60 Y-1718 6.20 3.25 6.25 10 0.50

[0052] For these alloys, in all cases the Cr content was 6.0 weightpercent, the Mo content was 1.5 weight percent, the Re content was 3weight percent, the columbium content was 1.5 percent, the carboncontent was 0.03 weight percent, and the boron content was 0.004 weightpercent. Three hundred pound heats of each of these alloys of Table IVwere prepared for evaluation.

[0053] Cyclic oxidation tests, with 20 cycles per hour for 103 hours to2200° F. and in a Mach 1.0 gas flow were performed and the weight lossesmeasured. The results are illustrated in FIG. 3.

[0054] Returning to the discussion of FIG. 2, the melted alloy issolidified to form an article, step 42. The solidification may be of anyoperable type, such as a multidirectional heat flow to produce anunoriented polycrystalline article, a substantially uniaxial directionalsolidification to produce a directionally oriented polycrystallinearticle, or a uniaxial solidification with a seed, constriction, orother approach to producing a substantially single crystal article.

[0055] The solidified article may optionally be post processed, step 44,by any operable approach. Post processing may include, for example,cleaning, coating, grinding, machining, and the like.

[0056] The approach just described has defined a low-tantalummodification of the baseline Rene™ N5 nickel-base superalloy.Low-tantalum modifications of other baseline nickel-base superalloys maybe made using the same principles. In one approach, a reduced-costnickel-base superalloy is selected by first identifying a baselinenickel-base superalloy having a nominal composition, in weight percent,comprising a baseline tantalum content of more than about 5 weightpercent tantalum, and a baseline sum (baseline hafnium content plusbaseline columbium content plus baseline titanium content plus baselinetungsten content), in weight percent. A number of baseline nickel-basesuperalloys are candidates for the application of the present approach,because of their high tantalum contents. Examples of such commercialbaseline nickel-base superalloys include PWA 1484 (nominally 8.7 percenttantalum), Rene™ 142 (nominally 6.35 percent tantalum), and the CMSXalloys such as CMSX-4 (nominally 6.5 percent tantalum) and CMSX-10(nominally 7.5 percent tantalum) may be modified according to theseprinciples to reduce their tantalum contents while maintainingacceptable properties.

[0057] A modified nickel-base superalloy is selected having a nominalcomposition, in weight percent, comprising a modified tantalum contentat least 1.5 weight percent less than the baseline tantalum content, anda modified baseline sum of (modified hafnium content plus modifiedcolumbium content plus modified titanium content plus modified tungstencontent) at least 1.5 weight percent greater than the baseline sum. Itis preferred that the increase in the sum of hafnium, columbium,titanium, and tungsten contents be at least as great as the decrease inthe tantalum content. It is also preferred that the modified nickel-basesuperalloy has a nonzero modified hafnium content, a nonzero modifiedcolumbium content, a nonzero modified titanium content, and a nonzeromodified tungsten content; that is, all of these elements should bepresent in nonzero amounts. The data also shows that the sum of themodified tungsten content plus a modified molybdenum content in themodified nickel-base superalloy should be at least about 6.5 weightpercent.

[0058] Other features and advantages of the present invention will beapparent from the following more detailed description of the preferredembodiment, taken in conjunction with the accompanying drawings, whichillustrate, by way of example, the principles of the invention. Thescope of the invention is not, however, limited to this preferredembodiment.

What is claimed is:
 1. An article comprising a composition consistingessentially of, in weight percent, from about 4 to about 12 percentcobalt, from about 3.5 to about 7 percent tungsten, from about 2 toabout 9 percent chromium, from about 0.5 to about 4.5 percent tantalum,from about 5.5 to about 7.5 percent aluminum, from 0 to about 5.5percent rhenium, from about 0.1 to about 1.2 percent titanium, from 0 toabout 3 percent molybdenum, from 0 to about 3 percent ruthenium, fromabout 0.5 to about 2 percent columbium, about 0.01 percent maximumboron, about 0.07 percent maximum carbon, from about 0.3 to about 1percent hafnium, about 0.01 percent maximum zirconium, about 0.03percent maximum yttrium, from 0 to about 0.5 percent vanadium, about0.01 percent maximum cerium, and about 0.01 percent maximum lanthanum,balance nickel and impurity elements.
 2. The article of claim 1, whereinthe article includes from about 3.0 to about 4.0 percent tantalum. 3.The article of claim 1, wherein the article includes from about 3.0 toabout 4.0 percent tantalum, from about 0.2 to about 0.4 percenttitanium, from about 0.5 to about 0.7 percent hafnium, and from about 1to about 2 percent columbium.
 4. The article of claim 1, wherein thearticle includes from about 6 to about 12 percent cobalt, from about 4.5to about 6.5 percent tungsten, from about 5.5 to about 6.5 percentchromium, from about 3.0 to about 4 percent tantalum, from about 5.8 toabout 6.3 percent aluminum, from about 2.8 to about 3.5 percent rhenium,from about 0.2 to about 0.4 percent titanium, from about 1.3 to about1.7 percent molybdenum, from about 0.5 to about 0.7 percent hafnium, andfrom about 1 to about 2 percent columbium.
 5. The article of claim 1,wherein the article includes from about 7 to about 10 percent cobalt,from about 6 to about 6.3 percent tungsten, about 6 percent chromium,from about 3.1. to about 3.5 percent tantalum, from about 5.9 to about6.3 percent aluminum, about 0.3 percent titanium, about 0.6 percenthafnium, about 3 percent rhenium, about 1.5 percent molybdenum, andabout 1.5 percent columbium.
 6. The article of claim 1, wherein thearticle is substantially a single crystal.
 7. The article of claim 1,wherein the article is a directionally oriented polycrystal.
 8. Thearticle of claim 1, wherein the article is shaped as a component of agas turbine engine.
 9. The article of claim 1, wherein the article isshaped as a gas turbine blade.
 10. A composition of matter consistingessentially of, in weight percent, from about 4 to about 12 percentcobalt, from about 3.5 to about 7 percent tungsten, from about 2 toabout 9 percent chromium, from about 0.5 to about 4.5 percent tantalum,from about 5.5 to about 7.5 percent aluminum, from 0 to about 5.5percent rhenium, from about 0.1 to about 1.2 percent titanium, from 0 toabout 3 percent molybdenum, from 0 to about 3 percent ruthenium, fromabout 0.5 to about 2 percent columbium, about 0.01 percent maximumboron, about 0.07 percent maximum carbon, from about 0.3 to about 1percent hafnium, about 0.01 percent maximum zirconium, about 0.03percent maximum yttrium, from 0 to about 0.5 percent vanadium, about0.01 percent maximum cerium, and about 0.01 percent maximum lanthanum,balance nickel and impurity elements.
 11. The composition of matter ofclaim 10, wherein the composition of matter includes from about 3.0 toabout 4.0 percent tantalum.
 12. The composition of matter of claim 10,wherein the composition of matter includes from about 3.0 to about 4.0percent tantalum, from about 0.2 to about 0.4 percent titanium, fromabout 0.5 to about 0.7 percent hafnium, and from about 1 to about 2percent columbium.
 13. The composition of matter of claim 10, whereinthe composition of matter includes from about 6 to about 12 percentcobalt, from about 4.5 to about 6.5 percent tungsten, from about 5.5 toabout 6.5 percent chromium, from about 3.0 to about 4 percent tantalum,from about 5.8 to about 6.3 percent aluminum, from about 2.8 to about3.5 percent rhenium, from about 0.2 to about 0.4 percent titanium, fromabout 1.3 to about 1.7 percent molybdenum, from about 0.5 to about 0.7percent hafnium, and from about 1 to about 2 percent columbium.
 14. Thecomposition of matter of claim 10, wherein the composition of matterincludes from about 7 to about 10 percent cobalt, from about 6 to about6.3 percent tungsten, about 6 percent chromium, from about 3.1. to about3.5 percent tantalum, from about 5.9 to about 6.3 percent aluminum,about 0.3 percent titanium, about 0.6 percent hafnium, about 3 percentrhenium, about 1.5 percent molybdenum, and about 1.5 percent columbium.15. A method for selecting a reduced-cost nickel-base superalloy, themethod comprising the steps of identifying a baseline nickel-basesuperalloy having a nominal composition, in weight percent, comprising abaseline tantalum content of more than about 5 weight percent tantalum,and a baseline sum (hafnium content plus columbium content plus titaniumcontent plus tungsten content), in weight percent, selecting a modifiednickel-base superalloy having a nominal composition, in weight percent,comprising a modified tantalum content at least 1.5 weight percent lessthan the baseline tantalum content, and a modified baseline sum of(modified hafnium content plus modified columbium content plus modifiedtitanium content plus modified tungsten content) at least 1.5 weightpercent greater than the baseline sum.
 16. The method of claim 15,wherein the step of selecting includes the step of selecting an absolutevalue of (the modified baseline sum minus the baseline sum) to be atleast as great as the absolute value of (the modified tantalum contentminus the baseline tantalum content).
 17. The method of claim 15,wherein the step of selecting includes the step of selecting themodified nickel-base superalloy to have a nonzero modified hafniumcontent, a nonzero modified columbium content, a nonzero modifiedtitanium content, and a nonzero modified tungsten content.
 18. Themethod of claim 15, wherein the sum of the modified tungsten contentplus a modified molybdenum content in the modified nickel-basesuperalloy is at least about 6.5 weight percent.
 19. A method forselecting a reduced-cost nickel-base superalloy, the method comprisingthe steps of identifying a baseline nickel-base superalloy having anominal composition, in weight percent, comprising a baseline tantalumcontent of more than about 5 weight percent tantalum, and a baseline sum(baseline hafnium content plus baseline columbium content plus baselinetitanium content plus baseline tungsten content), in weight percent,selecting a modified nickel-base superalloy having a nominalcomposition, in weight percent, comprising a modified tantalum contentat least 1.5 weight percent less than the baseline tantalum content, anda modified baseline sum of (modified hafnium content plus modifiedcolumbium content plus modified titanium content plus modified tungstencontent) at least 1.5 weight percent greater than the baseline sum,wherein an absolute value of (the modified baseline sum minus thebaseline sum) is at least as great as the absolute value of (themodified tantalum content minus the baseline tantalum content), whereinthe modified nickel-base superalloy has a nonzero modified hafniumcontent, a nonzero modified columbium content, a nonzero modifiedtitanium content, and a nonzero modified tungsten content, and whereinthe sum of the modified tungsten content plus a modified molybdenumcontent in the modified nickel-base superalloy is at least about 6.5weight percent.